In 1940 Karl Landsteiner and Alexander Wiener immunised rabbits and guinea pigs with rhesus macaque red blood cells and obtained a serum that agglutinated the red cells of most humans tested. The factor those cells shared was called Rh, and the terms Rh positive and Rh negative entered clinical use, becoming central to the management of haemolytic disease of the newborn.
Subsequent work showed the animal-derived antibody was recognising a different antigen, later named LW after Landsteiner and Wiener, rather than the human D antigen that actually defines Rh status. The naming was already established in transfusion practice and was not changed.
The macaque's role in research extends beyond that accident of nomenclature. It was used in polio vaccine development and in early spaceflight, and its genome was sequenced in 2007. Rhesus macaques are also among the few primates whose populations are increasing, largely through their success in human-dominated landscapes.